This invention relates to an improved method for the manufacture of insulated containers, particularly water heaters.
Electric and gas water heaters are generally comprised of a glass-lined, metal tank usually in the form of a cylinder which is surrounded by an insulating material retained about the tank by means of a protective, outer, sheet metal shell. In gas models, a burner compartment is normally located below the tank within the protective sheet metal shell. In electric models, a heating element normally projects through a side wall of the tank.
In order to improve the efficiency of such water heaters, it is desirable to provide external insulation for the tanks between the tank and the outer shell. Heretofore Fiberglas insulation has proven to be an excellent insulating material. More recently, expanded urethane foam has been utilized as the insulating material surrounding a water heater tank.
One of the problems associated with such a water heater assembly, particularly one having urethane foam as the insulating material, has been the method by which the foam material is formed about the tank. Generally the foam is injected as a liquid into the annular space between the inner tank and outer shell. However, it is necessary to use a core piece or other mold forming device within the space between the inner tank and outer protective shell to retain the foam within a desired region. For example, in an electric model the heating rod and controls are attached to the side wall of the tank and cannot be covered with foam.
One way of preventing foam from covering or forming over the controls has been to pack the region around the controls with Fiberglas insulation material. The Fiberglas insulation material then serves as a barrier during the foam molding operation.
Another procedure has been adopted by State Industries, Inc. of Ashland City, Tennessee. The State Industries procedure calls for forming a flexible plastic bag with an open top, closed sides and a closed bottom edge. The bag is then attached to and partially encircles the inner tank. Expandable urethane foam is next injected into the bag causing the bag to expand and fill the space between the inner tank and outer shell of the heater. In practice, the bag extends only partially around the circumference of the tank thus leaving a space or void in the region between the inner tank and outer shell defined by the opposite ends of the bag. Controls, inlets, outlets and the like then may be positioned in the void region. The spaces between the controls are appropriately packed with a Fiberglas insulation material.
The present invention is an improvement over the insulating and manufacturing methods described and permits utilization of expandable foam over substantially the entire outside surface area of a water heater tank. The method insures accurate and easily reproducible positioning of voids or spaces in the insulation through which controls, inlets, outlets and the like may be affixed to the tank. The method also eliminates usage of a large plastic bag from the manufacturing process and substitutes usage of a smaller, less expensive bag. As a result, undesired folds which may form in the large bag are avoided. Also, the insulating capability of the foam is enhanced since the foam will bond directly to the walls of the container and will not be retained in an air permeable bag. Finally, various distinct types of foam may be used in a single assembly to enhance insulation, fire retardancy and other features. These advantages, among others, are discussed in greater detail in following sections.